Bone remodeling
Encyclopedia
Bone remodeling is a lifelong process where mature bone tissue
is removed from the skeleton
(a process called bone resorption
) and new bone tissue is formed (a process called ossification
or new bone formation). These processes also control the reshaping or replacement of bone following injuries like fractures
but also micro-damage
, which occurs during normal activity. Remodeling responds also to functional demands of the mechanical loading.
In the first year of life, almost 100% of the skeleton is replaced. In adults, remodeling proceeds at about 10% per year.
An imbalance in the regulation of bone remodeling's two sub-processes, bone resorption and bone formation, results in many metabolic bone diseases, such as osteoporosis
.
requires close cooperation between these two types of cells. It relies on complex signaling pathways to achieve proper rates of growth and differentiation. These signaling pathways include the action of several hormones, including parathyroid hormone
(PTH), vitamin D
, growth hormone
, steroids, and calcitonin
, as well as several cytokines. It is in this way that the body is able to maintain proper levels of calcium
required for physiological processes.
Subsequent to appropriate signaling, osteoclasts move to resorb the surface of the bone, followed by deposition of bone by osteoblasts. Together, the cells that are responsible for bone remodeling are known as the basic multicellular unit (BMU), and the temporal duration (i.e. lifespan) of the BMU is referred to as the bone remodeling period
.
Osseous tissue
Osseous tissue, or bone tissue, is the major structural and supportive connective tissue of the body. Osseous tissue forms the rigid part of the bone organs that make up the skeletal system.-Formation:Bone tissue is a mineralized connective tissue...
is removed from the skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
(a process called bone resorption
Bone resorption
Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone fluid to the blood....
) and new bone tissue is formed (a process called ossification
Ossification
Ossification is the process of laying down new bone material by cells called osteoblasts. It is synonymous with bone tissue formation...
or new bone formation). These processes also control the reshaping or replacement of bone following injuries like fractures
Bone fracture
A bone fracture is a medical condition in which there is a break in the continuity of the bone...
but also micro-damage
Microdamage in bone
Microdamage in bone can be caused by the various loads to which bones are subjected during normal daily activity. It occurs in two different types mainly depending on the load: diffuse damage and micro-cracks....
, which occurs during normal activity. Remodeling responds also to functional demands of the mechanical loading.
In the first year of life, almost 100% of the skeleton is replaced. In adults, remodeling proceeds at about 10% per year.
An imbalance in the regulation of bone remodeling's two sub-processes, bone resorption and bone formation, results in many metabolic bone diseases, such as osteoporosis
Osteoporosis
Osteoporosis is a disease of bones that leads to an increased risk of fracture. In osteoporosis the bone mineral density is reduced, bone microarchitecture is deteriorating, and the amount and variety of proteins in bone is altered...
.
Physiology
The cells responsible for bone metabolism are known as osteoblasts, which secrete new bone, and osteoclasts which break bone down. The structure of bones as well as adequate supply of calciumCalcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
requires close cooperation between these two types of cells. It relies on complex signaling pathways to achieve proper rates of growth and differentiation. These signaling pathways include the action of several hormones, including parathyroid hormone
Parathyroid hormone
Parathyroid hormone , parathormone or parathyrin, is secreted by the chief cells of the parathyroid glands as a polypeptide containing 84 amino acids...
(PTH), vitamin D
Vitamin D
Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....
, growth hormone
Growth hormone
Growth hormone is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior...
, steroids, and calcitonin
Calcitonin
Calcitonin is a 32-amino acid linear polypeptide hormone that is producedin humans primarily by the parafollicular cells of the thyroid, and in many other animals in the ultimobranchial body. It acts to reduce blood calcium , opposing the effects of parathyroid hormone . Calcitonin has been found...
, as well as several cytokines. It is in this way that the body is able to maintain proper levels of calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
required for physiological processes.
Subsequent to appropriate signaling, osteoclasts move to resorb the surface of the bone, followed by deposition of bone by osteoblasts. Together, the cells that are responsible for bone remodeling are known as the basic multicellular unit (BMU), and the temporal duration (i.e. lifespan) of the BMU is referred to as the bone remodeling period
Bone remodeling period
In bone physiology, the bone remodeling period describes the temporal duration of the basic multicellular unit that is responsible for bone turnover...
.